Even in the era of multiple GNSSs, it is still a big challenge for standalone GNSS user equipment (UE) to provide seamless, timely and reliable positioning information in some extremely weak-signal environments because of the difficulties of GNSS UEs in either obtaining the ranging information or decoding the navigation (NAV) messages from the weak GNSS signals. The performance of GNSS UEs working in such environments is often quantified through the Time to First Position Fix (TTFF) and signal acquisition/tracking sensitivities, namely the lowest signal levels at which signals can be acquired and tracked, which are highly dependent on the UE startup modes (cold/warm/hot) that are categorized in terms of the accuracy of initial time, location and availability of ephemeris.
For warm start—the most typical start mode, the TTFF typically ranges from 18 to tens of seconds on a standalone GPS device, usually consisting of signal acquisition, tracking, bit synchronization, frame synchronization, ephemeris decoding, measurements taking and position computation, in which ephemeris decoding accounts for the most part of TTFF and is the mostly sensitive to signal strength. For better navigation experience with GNSS UEs, fast TTFF and high sensitivity are always desperately desired.
Assisted GNSS (AGNSS) and Ephemeris Extension (EE) are the primary technologies that are widely used to improve the TTFF and sensitivity, attempting to provide alternative ephemeris from ground (or even space-borne) assisting systems to UEs when the GNSS signals are too weak to allow timely downloading of the NAV messages from the GNSS satellites, along with additional aiding information to expedite signal acquisitions.
However, both the above technologies have significant limitations. For AGNSS, it relies badly on network connectivity. Considering the life cycle of the aiding data from the AGNSS server, the UEs have to remain connected to the network so that the aiding data can be updated a certain period of time (say 2 hours for Assisted GPS (AGPS)). For EE, although the provided aiding data has longer life expectancy (for days), it can be hardly used in assisting tracking loop due to the difference between extended ephemeris and the actual ephemeris broadcast from the signals.